It is well-known to use photografting to modify the surface of films, sheets and molded objects formed from polyolefins. For example, the publication of Tazuke and Kimura (1978) discloses photografting onto poly(propylene), poly(ethylene) and several other polymer films using benzophenone as a sensitizer. In this publication the choice of solvent and sensitizer was noted to be very important. The publication of Ang et al (1980) discloses an irradiation procedure where the sensitizer is dissolved in the hydrophilicitizing agent solution and can be used for the photosensitized copolymerization in high yields of styrene, 4-vinyl pyridine and methyl methacrylate to poly(propylene). Again, this publication notes that the reaction was found to be very specific to certain types of sensitizers.
The publication of Ogiwara et al (1981) discloses the photografting on poly(propylene) and low-density poly(ethylene) (LDPE) films on which sensitizers were coated beforehand. The sensitizers coated on films enabled vinyl hydrophilicitizing agents, such as methyl methacrylate, acrylic acid and methacrylic acid to graft easily with high yields. The hydrophilic hydrophilicitizing agents acrylic acid and methacrylic acid were conveniently grafted using them in aqueous solution in a liquid phase system. The publication of Allmer et al (1988) discloses the modification of surfaces of LDPE, high-density poly(ethylene) (HDPE) and polystyrene by grafting with acrylic acid. The grafting is performed in the vapor-phase and increased the wettability of the polymer. It was observed that acetone was able to initiate grafting and was found to promote and direct grafting to the surface. The publication of Allmer et al (1989) discloses the grafting of the surface of LDPE with glycidyl acrylate and glycidyl methacrylate by photoinitiation. Acetone and ethanol were used as solvents, with acetone yielding slightly more grafting at the surface.
The publications of Yao and Ranby (1990a, 1990b and 1990c) disclose inter alia a process for the continuous photoinitiated graft copolymerization of acrylamide and acrylic acid onto the surface of HDPE tape film. The process is performed under a nitrogen atmosphere using benzophenone as the photoinitiator. It was noted that pre-soaking was very important for efficient photographing within short irradiation times. The application of this pre-soaking photografting method to poly(ethylene terephthalate) (PET) was also disclosed. In this context acetone was found to be a somewhat better solvent than methylethyl ketone and methylpropyl ketone. When applied to a continuous process for the photochemically induced graft polymerization of acrylamide and acrylic acid of poly(propylene) (PP) fiber surface under a nitrogen atmosphere, optimal concentrations of hydrophilicitizing agent and initiator in the pre-soaking solution were determined.
The publications of Kubota and Hata (1990a and 1990b) disclose an investigation of the location of methacrylic acid chains introduced into poly(ethylene) film by liquid and vapor-phase photograftings and a comparative examination of the photografting behaviours of benzil, benzophenone and benzoin ethyl ether as sensitizers. In these latter studies poly(methacrylic acid) was grafted onto initiator-coated LDPE film.
The publication of Edge et al (1993) discloses the photochemical grafting of 2-hydroxyethyl methacrylate (HEMA) onto LDPE film. A solution phase method is used to produce a material with increased wettability. The publication of Singleton et al (1993) discloses a method of making a polymeric sheet wettable by aqueous solvents and useful as an electrode separator in an electrochemical device. The polymeric sheet is formed from fibers which comprise poly(propylene) alone and is distinguished from a membrane formed from a microporous polymer sheet. The publication of Zhang and Ranby (1993) discloses the photochemically induced graft copolymerisation of acrylamide onto the surface of PP film. Acetone was shown to be the best solvent among the three aliphatic ketones tested.
The publications of Yang and Ranby (1996a and 1996b) disclose factors affecting the photografting process, including the role of far UV radiation (200 to 300 nm). In these studies benzophenone was used as the photoinitiator and LDPE film as the substrate. Added water was shown to favour the photografting polymerisation of acrylic acid on the surface of polyolefins, but acetone was shown to have a negative effect due to the different solvation of poly(acrylic acid) (PAA).
The publication of Hirooka and Kawazu (1997) discloses alkaline separators prepared from unsaturated carboxylic acid grafted poly(ethylene)-poly(propylene) fiber sheets. Again, the sheets used as a substrate in these studies are distinguished from a membrane formed from a microporous polymer sheet.
The publication of Xu and Yang (2000) discloses a study on the mechanism of vapor-phase photografting of acrylic acid in LDPE. The publication of Shentu et al (2002) discloses a study of the factors, including the concentration of hydrophilicitizing agent, affecting photo-grafting on low-density LDPE. The publication of El Kholdi et al (2004) discloses a continuous process for the graft polymerisation of acrylic acid from hydrophilicitizing agent solutions in water onto LDPE. The publication of Bai et al (2011) discloses the preparation of a hot melt adhesive of grafted low-density poly(ethylene) (LDPE). The adhesive is prepared by surface UV photografting of acrylic acid onto the LDPE with benzophenone as the photoinitiator.
The publication of Choi et al (2001) states that graft polymerisation is considered as a general method for modifying the chemical and physical properties of polymer materials.
The publication of Choi (2002) discloses a method for producing an acrylic graft polymer on the surface of a polyolefin article comprising the steps of immersing the article in a solution of an initiator in a volatile solvent, allowing the solvent to evaporate, and then immersing the article in a solution of an acrylic hydrophilicitizing agent before subjecting the article to ultraviolet irradiation in air or an inert atmosphere. Acrylic acid is used as the acrylic hydrophilicitizing agent in each one of the Examples disclosed in the publication, although the use of equivalent amounts of methacrylic acid, acrylamide and other acrylic hydrophilicitizing agents is anticipated.
The publication of Choi (2004) discloses the use of “ethylenically unsaturated hydrophilicitizing agents” in graft polymerisation. These other hydrophilicitizing agents are disclosed as hydrophilicitizing agents that are polymerisable by addition polymerisation to a thermoplastic polymer and are hydrophilic as a consequence of containing carboxyl (—COOH), hydroxyl (—OH), sulfonyl (SO3), sulfonic acid (—SO3H) or carbonyl (—CO) groups. No experimental results concerning the chemical and physical properties of graft polymers prepared by a method using these other hydrophilicitizing agents is disclosed.
The publication of Choi (2005) discloses a non-woven sheet of polyolefin fibres where opposed surfaces of the sheet are hydrophilic as a consequence of an acrylic graft polymerisation. The properties of the sheet are asymmetric, the ion exchange coefficient of the two surfaces being different. The method used to prepare these asymmetric acrylic graft polymerised non-woven polyolefin sheets comprises the steps of immersing the substrates in a solution of benzophenone (a photoinitiator), drying and then immersing the substrate in a solution of acrylic acid prior to subjecting to ultraviolet (UV) irradiation. The irradiation may be performed when the surfaces are in contact with either air or an inert atmosphere.
The publication of Gao et al (2013) discloses a method of preparing a radiation cross-linked lithium-ion battery separator. In an example, a porous polyethylene membrane is immersed in a solution of benzophenone and triallyl cyanurate in dichloromethane. The immersed membrane is dried at room temperature before being immersed in a water bath at 30° C. and irradiated on both sides using a high-pressure mercury lamp for three minutes.
The objective of the majority of these prior art methods is to improve the adhesion, biocompatibility, printability or wettability of the surface of a substrate. These improvements to surface characteristics are to be distinguished from the use of UV-initiated polymerisation to modify the permeability of preformed microporous polyolefin substrates, such as the substrates described in the publications of Fisher et al (1991) and Gillberg-LaForce (1994).
It is well-known to prepare thin film composite membranes to modify the permeability of a preformed microporous polyolefin substrate. For example, the publication of Jones (1990) discloses a composite permselective membrane comprising an ultrathin semipermeable layer comprising a polybenzimidazole polymer in occluding contact with at least one surface of a microporous polymer support layer. The membranes are asserted to provide better combinations of flux and rejection rates in reverse osmosis processes than do conventional semipermeable membranes of polybenzimidazole polymer alone.
The publication of Callahan and Johnson (1990) discloses a composite membrane having a microporous support which is coated with a UV curable polymer composition having a sufficiently high viscosity to prevent pore filling upon coating and curing.
The publication of Gillberg-LaForce and Gabriel (1991) discloses a pore modified microporous membrane which is made by a process of incorporating a polymerizable vinyl hydrophilicitizing agent within the pores of a microporous membrane followed by polymerization to secure the resulting polymer within the pores. The process is stated to be particularly suitable for modifying a hydrophobic microporous membrane with a hydrophilic polymer, as occurs for example when polyacrylic acid is secured into the pores of a polypropylene microporous membrane.
The publication of Callahan and Johnson (1992) discloses a composite membrane having a microporous support which is coated with a UV curable polymer composition having a sufficiently high viscosity to prevent pore filling upon coating and curing.
The publication of Cussler et al (1992) discloses a process for modifying the properties of a hydrophobic microporous membrane which includes the steps of treating a hydrophobic microporous membrane with a surfactant to render the membrane hydrophilic, permeating the membrane with a polyol, and crosslinking the polyol to yield a hydrophilic microporous membrane having pores filled with an aqueous gel. The modified membranes are asserted to be useful in carrying out chromatographic separations.
The publication of Donato and Phillips (1993) discloses a composite membrane having a microporous support which is coated with a polymer selected from the group consisting of polyethylene oxide, polyacrylic acid, poly(methyl methacrylate) and polyacrylamide wherein there is no pore filing of the microporous support. The publication of Donato (1994) discloses a composite membrane having a microporous support coated with an aqueous polyeurethane dispersion composition. The publication of Donato and Phillips (1994) discloses a composite membrane having a microporous support which is coated with a polymer composition and a contact adhesive layer applied to said polymer.
For the most part, the methods of preparing composite membranes disclosed in these publications use UV initiated polymerisation to form polymers in situ. Methods of adhering dissimilar preformed polymers to the surface of the microporous polyolefin substrates are less well known.
It is an object of the present invention to provide a method of decreasing the hydrophobicity of preformed microporous poly(ethylene) sheets and thereby provided modified microporous polyethylene sheets suitable for use in the preparation of water permeable asymmetric composite membranes. It is an object of the present invention to provide an asymmetric composite membrane suitable for use in the recovery or removal of water from dairy and other feed streams. These objects are to be read in the alternative with the object at least to provide a useful choice.